Gene knock-out mice have become important models for delineating molecular and functional roles of specific genes. In contrast with a number of other apolipoproteins, ApoD is expressed in the brain as well as other tissues. Its metabolism is abnormal in the spontaneously occurring murine anolog of Type C Niemann-Pick disease of humans. It would be informative to produce ApoD -/- mice to determine their phenotype and to cross-breed them with Niemann-Pick C mice. In addition, ApoD may serve as a reservoir of cholesterol and other myelin lipids in demyelinating conditions. Its role in remyelination and nerve regeneration will be explored in the ApoD -/- mice. In an attempt to generate such models for neurological disorders, we have initiated studies to disrupt the ApoD gene in mouse embryonic stem cells. We have used rat ApoD cDNA probe to screen the 129/svj mouse genomic library. Eight genomic clones of ApoD have been isolated and characterized. An ApoD targeting construct has been engineered for positive/negative selection. ApoD cDNA has been cloned by RT-PCR from mouse kidney RNA.